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Step-by-Step Explanation
1. Understanding the Float Glass Process
In the float glass process, molten glass is allowed to spread and form a flat, even sheet by floating it over a liquid metal. The key requirement is that the metal used must remain in its liquid state at the temperature where glass is still molten, ensuring a smooth surface without solidifying prematurely.
2. Properties Needed in the Metal
The chosen metal must have:
A melting point that is lower than or close to the temperature at which glass is molten.
A high density, so the molten glass can easily float on top without mixing.
A relatively inert nature, so it does not react extensively with components of the glass.
3. Why Mercury is the Correct Answer
Among the given options—tin, sodium, magnesium, and mercury—only mercury remains liquid at or near room temperature (its melting point is about $-39^\circ \text{C}$). Hence, it does not solidify before the glass cools. While tin is also used in modern industrial float glass processes, mercury fits the traditional description due to its liquid state at room temperature, allowing the glass to float and form a glossy, uniformly flat mirror surface.
4. Conclusion
Therefore, for obtaining mirror plates using the float method, mercury is a suitable metal since it remains in liquid form and provides a smooth surface upon which the molten glass can spread out, eventually forming high-quality mirror plates.
